1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine and a printer, for performing electrophotographic image forming processing by forming an electrostatic latent image on the surface of an image carrier, and transferring a toner image electrostatically adhering to the electrostatic latent image onto a transfer material.
2. Description of the Related Art
An image forming apparatus performing electrophotographic image forming processing has been adapted to meet the demands of high resolution products in recent years by reducing the layer thickness of the photosensitive layer covering the surface of the image carrier to as thin as 10 to 20 xcexcm. With the image carrier covered by the photosensitive layer with a thinner film thickness, a potential is smoothed in each of a light portion and a dark portion, thereby making it possible to form an electrostatic latent image with a well-defined edge portion. As a result, when the electrostatic latent image is developed into a visible image, a sharp toner image can be obtained.
However, when the toner image developed as the visible image from the electrostatic latent image with the well-defined edge portion is transferred onto a transfer material, a part of toner forming the toner image is scattered over the image carrier, which results in a problem that the image quality on the transfer material is deteriorated. It is assumed that this is because reducing the film thickness of the photosensitive layer covering the image carrier causes an abrupt change in potential at the edge portion of the electrostatic latent image, and an electric field in a transfer electric field acting on in a direction toward the transfer material is readily disturbed. Also, when toner with excellent transfer efficiency is used to reduce an amount of toner remaining after the transferring step, the toner is more easily scattered with a disturbance of the electric field in the transfer electric field.
Accordingly, Japanese Unexamined Patent Publication JP-A 2000-75690 (2000) discloses an arrangement provided with means for reducing a potential difference between an image portion where toner is adhered and a non-image portion where toner is not adhered on the surface of the image carrier after the developing step and before the transferring step, so that even when an electrostatic latent image with a well-defined edge portion is formed on the photosensitive layer with a thinner film thickness, a transfer electric field that acts on a toner image at a transfer position is allowed to remain intact, whereby it is possible to obtain a satisfactory transferred image without causing the scattering of toner.
The arrangement disclosed in JP-A 2000-75690 supra describes attenuation of a potential at the non-image portion by 20% to 60% as the means to reduce the potential difference between the image portion and non-image portion on the image carrier before the transferring step. However, the thicknesses of the photosensitive layer and toner layer, and other parameters that regulate the transfer electric field, such as amounts of charge and dielectric constants of these layers, are not considered at all, which poses a problem that when a use environment or the like changes, the scattering of toner cannot be prevented in a reliable manner.
It is therefore an object of the invention to provide an image forming apparatus for adequately setting the thicknesses of the photosensitive layer and toner layer that exert influence upon the development of a transfer electric field, and parameters that regulate the transfer electric field, such as amounts of charge and dielectric constants of these layers, so that even when a use environment or the like changes, the transfer electric field that acts on a toner image at a transfer position is allowed to remain intact constantly, whereby the scattering of toner at the time of transfer can be prevented in a stable manner, and therefore, a transferred image with a satisfactory image quality can be obtained in a reliable manner.
The invention has the following arrangements as means to solve the problems.
The invention provides an image forming apparatus comprising:
an image carrier including a photosensitive layer,
on the photosensitive layer an electrostatic latent image being formed through a photoconductive function,
a toner layer being formed on a surface of the image carrier by causing electrostatic adhesion of toner particles to the electrostatic latent image; and
transfer means for transferring the toner particles electrostatically adhering to the surface of the image carrier, to a surface of a recording medium with an electrostatic force by bringing the surface of the recording medium into contact with the surface of the image carrier through the toner layer and applying a transfer voltage from the transfer means to another surface of the recording medium,
wherein parameters of equivalent air thicknesses topc (xcexcm) and yt (xcexcm) of the photosensitive layer and toner layer, respectively, an amount of charge per unit area qpm ((mg/cm2)xe2x88x921) of the toner layer, the amount of charge per unit area being normalized based on an amount of charge per unit area xcfx81opc in a non-image portion of the photosensitive layer, an amount of charge per unit area xcfx81xe2x80x2opc in the non-image portion of the photosensitive layer upon entrance into a transfer region, and a total amount of charge per unit area xcfx81o of the toner layer satisfy the following Equation (1):
|(xcfx81xe2x80x2opcxe2x88x92xcfx81o)/(xcfx81opcxe2x88x92xcfx81o)| less than 1
where
xcfx81o=qpm{(xe2x88x920.01xc2x7topc+0.145)yt2+(0.001xc2x7topc+0.937)yt}.xe2x80x83xe2x80x83(1)
According to the invention, the dielectric constant and thicknesses topc and yt of the photosensitive layer and toner layer, respectively, the amount of charge per unit area in the non-image portion of the photosensitive layer xcfx81opc, the amount of charge per unit area of the toner layer qpm, the total amount of charge per unit area xcfx81o of the toner layer, and the amount of charge per unit area xcfx81xe2x80x2opc in the non-image portion of the photosensitive layer are set upon entrance into the transfer region so as to satisfy the relation expressed by Equation (1). Hence, all the parameters that regulate an electric field developed in the transfer region are set so as not to increase an amount of variance of transfer electric field, there by making it possible to prevent the scattering of toner at the time of transfer in a stable manner.
In the invention it is preferable that the parameters satisfy the following equation:
|(xcfx81xe2x80x2opcxe2x88x92xcfx81o)/(xcfx81opcxe2x88x92xcfx81o)| less than ⅓.
According to the invention, variance of electric field in the transfer region by the transfer electric field is controlled to stay below ⅓. Hence, the scattering of toner of the toner layer over the non-image portion of the photosensitive layer is further suppressed, and deterioration of an image quality is further reduced. In other words, by limiting the variance of electric field to stay below ⅓, based on the fact that the psychological recognition level of visual stimulus by the human sense of sight and the intensity of the visual stimulus are in a logarithmically functional relation, the user senses that the scattering of the toner becomes half.
In the invention it is preferable that in Equation (1):
xcfx81xe2x80x2opc≈xcfx81o.
According to the invention, an electric field distribution is smoothed on the surface of the image carrier positioned at the transfer region. Hence, an electric field developed in a direction along the surface of the image carrier is controlled, and therefore, it is possible to prevent the scattering of the toner particles electrostatically adhering to the surface of the image carrier.
In the invention it is preferable that a positive value is given to a quotient when variance of electric field dE expressed as the following Equation (2) is divided by xcfx81opc:
xe2x80x83dE=(xcfx81xe2x80x2opcxe2x88x92xcfx81o) (xe2x88x920.0248xc2x7yt+1.94)qpmxc2x7topcxe2x80x83xe2x80x83(2).
According to the invention, because the variance of electric field dE is given with a positive value, an electric field heading inward to the center of the toner layer in a direction along the surface of the image carrier is formed in the transfer region. Hence, no static force heading outward from the toner layer acts on the toner particles electrostatically adhering to the surface of the image carrier in the transfer region, and therefore, the toner particles are not scattered over the non-image portion.
In the invention it is preferable that the image forming apparatus further comprises charge providing means for providing charges to lessen variance of transfer electric field in the transfer region to the surface of the image carrier after the electrostatic latent image is developed into a visible image and before the surface reaches the transfer region.
According to the invention, all the parameters that regulate the electric field developed in the transfer region are set so as not to increase an amount of variance of transfer electric field, plus charges that lessen the variance of electrostatic field are provided to the surface of the image carrier before the transfer is effected. Hence, the electric field distribution is smoothed further on the surface of the image carrier positioned at the transfer region, and the electric field developed in the direction along the surface of the image carrier is better controlled, thereby making it possible to prevent the scattering of the toner particles electrostatically adhering to the surface of the image carrier in a more stable manner.
In the invention it is preferable that the charge providing means is means for providing charges of a polarity the same as a polarity of charge of the toner or means for providing charges of a polarity opposite to a polarity of charge of the photosensitive layer on the image carrier.
According to the invention, charges are provided intensively to the toner that forms a valley of the electric field distribution in the transfer region, or the charges collected in the non-image portion that forms a peak of the electric field distribution are neutralized. In either case, the electric field distribution can be smoothed, thereby making it possible to prevent the scattering of toner by lessening variance of electric field.